Devices such as cold cathode tube and the like have previously been used as light source for a lighting system or a liquid crystal display backlight. Recently, pseudo-white light sources have been developed as an alternative light source, which is a combination of a light source emitting blue light and materials emitting yellow light as a result of absorbing blue light. In this pseudo-white light source, an InGaN base light-emitting diode is used as light source emitting blue light and cerium-activated yttrium aluminate is used as a material emitting yellow light, for example.
However, color spectrum generated by pseudo-white light sources is essentially deficient in green light component and red light component and, therefore, these pseudo-white light sources were poor in their color rendering and color reproduction property. In order to solve this problem, proposals have been made to improve the light rendering property and light reproduction property of these pseudo-white light sources. These proposals include modification of yttrium aluminate complex so that it emits yellowish green light, or further addition of such substances as to absorb blue light and emit red light to the yttrium aluminate complex so as to supplement red light component of the pseudo-white light source.
In many cases, however, red luminescent materials absorb not only blue light but also green light or yellow light whose wavelength is longer than blue light but shorter than red light. As such substances can be cited europium activated alkali earth metal sulfides, europium activated alkali earth metal and silicon nitrides, and europium activated alkali earth metal and silicon oxynitrides. These substances usually absorb light of 400 nm to 580 nm wavelength efficiently and emit orange to red light with peak wavelength of 580 nm to 680 nm.
Orange to red luminescent materials, typified by the above-mentioned ones, absorb shorter wavelength, green to yellow light. Therefore, combined use of orange to red luminescent materials and green to yellow luminescent materials results in partial absorption of green to yellow light component by orange to red luminescent materials, and this causes a marked decrease in luminous flux of light emitting device.
At present, attempts are being made to prevent the loss of luminous flux caused by absorption of short wavelength light by luminescent materials emitting long wavelength light. Patent Document 1 is one of such examples. In this patent, a light emitting device comprises two kinds of materials (here called, “material A” and “material B”), which absorb light from a light source and emit light of different wavelength. It is so arranged that, when the material A (which corresponds to a material emitting orange to red light) absorbs a part of the light emitted from the material B (which corresponds to a material emitting green to yellow light), the improvement in color rendering and the prevention of luminous flux loss can be achieved by locating the material A closer to the light source than the material B is.
In addition, display devices have previously been used which visualize clearly images formed on the image formation unit by irradiating light (backlight) directed against the image formation unit, with some images formed thereon, from behind. Display devices of this kind include: liquid crystal display using a liquid crystal unit as image formation unit, indoors indicator (emergency exit lamp, traffic signal and the like) with its sign (image formation unit) illuminated with light originated inside.
These display devices usually comprise a backlight unit which radiates light from behind against the image formation unit. Fluorescence lamp and cold cathode tube have previously been used as this kind of backlight unit.
The problem is, fluorescent lamps and cold cathode tubes, used as backlight unit, are difficult to reduce in size of the backlight unit and, besides, their lifetime is rather short.
Another concern is that these devices contain mercury in them, which may exert an undesirable effect on the environment, making their use even more difficult.
In recent years, a proposal has been made to design, as backlight unit, a light emitting device which makes use of a light source and fluorescent materials capable of absorbing light from the light source and emitting fluorescence. For example, the art as mentioned above, which utilizes pseudo-white light source as backlight unit using light-emitting diode of InGaN system and cerium-activated yttrium aluminate as light source and luminescent materials, respectively.
Another proposal is to use a light emitting device proposed in Patent Document 1 as backlight.
[Patent Document 1] Japanese Patent Laid-Open Publication (Kokai) No. 2004-71726